The manufacturers of integrated circuits typically specify maximum operating temperature for reliable operation. Exceeding the maximum specified temperature may damage the integrated circuit, and would affect the long term reliability of the integrated circuit. It is therefore desirable to keep the integrated circuit operating temperature below the maximum operating temperature of the device.
Heat pipe apparatus have been used to transfer the heat energy away from the integrated circuit or integrated circuit package. A basic heat pipe apparatus is comprised of four elements: a thermally conductive material attached to the exposed surface of an integrated circuit (or integrated circuit package), a pipe element connecting the thermally conductive material to a cooling mechanism, and a liquid contained within the pipe element. As the temperature of the integrated circuit increases, it heats the liquid to its boiling point and the liquid evaporates. As the resultant vapor passes along the length of the pipe, it is cooled by the cooling mechanism, condenses, and flows back towards the integrated circuit. The section of the device where the heating of the liquid takes place is referred to as the evaporator section, while the section of the device where the vapor reaches the cooling mechanism and is condensed is the condenser section.
A variety of methods is used for conveying the liquid between the evaporator and condenser sections. A passive method referred to as thermo-syphon cooling utilizes the natural forces of gravity and buoyancy in a heat pipe apparatus. In thermo-syphon heat pipe apparatus, the condenser section is positioned above the evaporator section. The buoyancy of the vapor causes it to rise naturally upwards towards the condenser section. The condensate is returned to the evaporator section by gravitational forces. Thermo-syphon cooling provides a passive and inexpensive method for heat dissipation.
Another consideration arises in systems having many integrated circuits or integrated circuit packages assembled on a planar circuit board which are to be associated with a common cooling device. If the integrated circuits or integrated circuit packages do not all have the same height, the distances from the tops of the integrated circuit packages to the cooling apparatus differ from each other. One method of resolving the height differential problem is to use a resilient bellows pipe element between each integrated circuit package and a common condenser device. One example of a heat pipe apparatus for cooling integrated circuits in a multi-chip assembly which uses a bellows pipe element in this manner is discussed in U.S. Pat. No. 4,951,740 issued Aug. 28, 1990 to Peterson et. al. In Peterson's system each bellows pipe element communicates with the common condenser at its top end and is attached to an integrated circuit at its bottom end. The interior bottom surface of the bellows device is lined with a wick saturated with a working fluid. This section of the bellows device is the evaporator section. Wicks also line the interior sidewalls of the bellows, and a wick extends longitudinally within the bellows. Each bellow device communicates with a corresponding condenser section. The condenser subsections are interconnected to allow the vapor pressure in the entire condenser section to equalize, enabling uniform longitudinal activity of the condenser section. Each condenser subsection is lined with wicks which extend into the associated bellows device.
During integrated circuit operation, the integrated circuit or integrated circuit package generates heat energy which causes the working fluid in the evaporator section to vaporize and rise towards the condenser subsection. The vapor condenses at the condenser subsection and is returned to the evaporator section of the bellows device along the internal wick pathways.
Because Peterson's invention involves a sealed system, there is a potential for non-condensable vapor to be trapped in the condenser subsection, causing the condenser section to act as a heat insulator. The condenser wicks absorb the non-condensable vapor to help to prevent this potential hazard. In addition, because the saturated wick reservoir in the bottom of the bellows device has a relatively small capacity, condensed working fluid must be rapidly returned to the wick reservoir for re-evaporation and maintenance of cooling system equilibrium. The addition of the longitudinal wick and the interior sidewall wicks provide controlled pathways for liquid return to the wick reservoir.
A thermal problem which can be encountered in heat pipe design revolves around the connection between the integrated circuit and the heat pipe apparatus. When a heat pipe apparatus is rigidly attached with epoxy or solder, there is the potential danger that hot spots from the integrated circuit could cause the evaporator section of the bellows device to expand unevenly. It is desirable to protect against hot spots which could cause uneven expansion between the bellows device and the integrated circuit. Uneven expansion could potentially eliminate contact between parts of the two surfaces and the resulting insufficient cooling could damage the integrated circuit. Peterson's design addresses the problem of thermal expansion between the heat pipe apparatus and the integrated circuit with wicks which line the bottom of the evaporator section of the bellows device. The wick section spreads fluid evenly and is intended to prevent the formation of hot spots in the area where the bellows contacts the integrated circuit.
It would be desirable from a manufacturing standpoint to provide a cooling system which would eliminate the use of expensive wicks while relieving the adverse effects arising from thermal expansion discussed above. Further, it would be desirable to provide a wickless cooling system in which the evaporator device is distributed among many heat pipe apparatus in a multi-chip assembly.